1. Field of the Invention
The present invention relates to a semiconductor photodetector for detecting incident light as first disclosed in Japanese Patent Application No. 5067/1993 filed Jan. 14, 1993, the contents of which are incorporated herein by reference.
2. Related Background Art
Currently, an In.sub.1-x Ga.sub.x As mixed crystal is well known as one of mixed crystal semiconductor materials that may be used for photodetection. By controlling the composition ratio of this mixed crystal semiconductor to change the forbidden bandgap, this mixed crystal can be used as the light-absorbing layer of a photodetector for optical communication, a spectrophotometer, etc. When the composition ratio of this In.sub.1-x Ga.sub.x As mixed crystal is changed, a mixed crystal semiconductor having various bandgaps ranging from that of InAs=0.36 eV to that of GaAs=1.42 eV can be made. For example, with x=0.35, In.sub.0.65 Ga.sub.0.35 As has a forbidden bandgap=0.48 eV. This mixed crystal can be used as a light-absorbing layer to manufacture a photodetector having a sensitivity up to a wavelength of 2.0 .mu.m. As the value x becomes smaller, a photodetector having a sensitivity to a longer wavelength can be made.
However, when the composition ratio of the mixed crystal is changed, not only the bandgap but also the lattice constant changes. When x=0.35, the lattice constant =5.9166 .ANG. which is substantially different from that of InP=5.8688 .ANG.. Generally, when a crystal having a lattice constant substantially different from that of a substrate is grown on the substrate, a lot of dislocations are induced from its interface to degrade the crystalline quality. For this reason, no crystal suitable for practical use can be obtained. However, when the difference between the lattice constants is very small, a relatively satisfactory epitaxial layer can be obtained. By using this, the composition ratio of the mixed crystal is changed in the growing direction of the epitaxial layer to gradually change the lattice constant, thereby finally growing an epitaxial layer having a composition ratio with a desired lattice constant, i.e., bandgap. These layers are called a graded layer as a whole. When a photodetector is to be manufactured, a light-absorbing layer and a capping layer are sequentially epitaxially grown on the surface of the graded layer. Conventionally, to maintain the good crystallinity of the light-absorbing layer, a composition ratio is applied such that the lattice constants of the uppermost layer of the graded layer and the capping layer are equalized with that of the light-absorbing layer.
The conventional semiconductor photodetector has the above structure. With this structure, however, since a dark current is large, weak incident light cannot be detected with high accuracy.